1. Field of the Invention
The invention relates to a pyrolysis furnace and a method of high temperature cracking using the same, more specifically, relates to a pyrolysis furnace with new type heat supply for high temperature cracking reaction of hydrocarbons and the method of high temperature cracking using the same.
2. Description of the Prior Art
As is known to all, the pyrolysis reaction of hydrocarbons is main means for production of very important industrial raw materials such as ethylene, propylene, etc. Even if a little improvement occurs in this field, it can bring about giant economic and social benefits. Pyrolysis furnace is the main equipment for performing high temperature cracking, therefore, nearly all chief hydrocarbons and petro-chemical companies of the world pay great attention to and not hesitate to make huge amount of investment for modification of pyrolysis furnaces.
As is understood to a person skilled in the art, high temperature condition of cracking reaction is achieved by heat supply from burners to radiation tubes in the radiant section. According to the mounting location in the radiant section, the burners are sorted into bottom burners, wall burners, and top burners, where in the bottom and top burners employ both gas and liquid fuel for burning, as well as they are also in the form of gas-liquid combined burners, Now, there are 3 kinds of arrangement of radiant tubes, namely, single row, double row and staggered row. Above mentioned content is available from reference to <<The technology of ethylene>>, by Chen Bing, (chemical industrial pub. house, 1997. chapter 4)
In U.S. Pat. No. 4,361,478 the campany LINDE discloses a Pyrolysis furnace entirely employing heat supply by means of wall burners. the pyrolysis furnace solely employing heat supply by means of wall burners has the feature of uniform temperature distribution in furnace chamber, small width of furnace chamber, etc, but too many burners are located in whole pyrolysis furnace, distribution piping of fuel is complicated, investment is large, and the operation and maintenance in practice is an expensive matter.
In U.S. Pat. No. 4,999,089 the Japanese company MITSUI discloses a pyrolysis furnace entirely employing heat supply by means of top burners. At that invention, the structure of radiant section is irregular, the form of fuel current in furnace chamber is complicated, moreover, the heated wall at radiant section is tilted, under the condition of high temperature operation, the isolation lining materials of heater wall are easy to be break-up in operation, it results in great amount of repair. Be walls, the outlet of fuel gas is located at bottom portion of radiant section, the high-powered extraction fan is needed for back current of fuel gas in radiant section, such furnace has increased cost and energy consumption.
In U.S. Pat. No. 5,151,158, the company Stone & Webster describes a pyrolysis furnace with entire heat supply from bottom burners, it wholly use heat supply from bottom burners. it is advantageous in simple operation and minimum amount of maintenance. But the requirement to burners is relative high, when the height of furnace chamber at radiant section is higher, it is necessary to have some burners of special design to meet uniform temperature requirement in furnace chamber. These burners are complex in manufacture and expensive.
In U.S. Pat. No. 4,342,642 the company LUMMUS discloses a pyrolysis furnace with bottom-wall-combined heat supply, although this kind of heat supply can partly compensate the shortage of a low height of flame of entire bottom burners, complexity of distribution piping and poor flexibility of fuel of entire wall burners, it still need for appropriate wall-to bottom burning ratio to satisfy temperature uniformity of furnace chamber. Moreover, this heat supply still based upon side wall burners, wherein a series of problems, such as complicated fuel distribution piping, large investment, complicated operation, and difficult maintenance are still presentation.
Above mentioned pyrolysis furnace of prior art generally comprising: a convection section, used for preheating the hydrocarbons feed stock; a radiant section, used for high temperature cracking hydrocarbons feedstock; and a crossover section, connected between the convection section and radiant section. A typical pyrolysis furnace with bottom burners is shown in FIG. 5, wherein bottom burners 8 and radiant tubes 7 are arranged in a radiant section 3, a convection section 2 is located above the radiant section and axially shifted, wherein convection tubes 1 are arranged, a crossover section 6 is passed horizontally from top portion of radiant section 3 to connect with bottom portion of convection section 2. The above mentioned pyrolysis furnace of prior art has greater overall height, it increases design and technology difficult and results in larger amount of capital expenditure.
Besides, the structure and arrangement of radiant tubes is another factor affecting the cracking reaction result. The radiant section of traditional vertical pyrolysis furnace inmost cases employs single row tubes to ensure uniform heat receipt of radiant tubes. There are some companies, for the purpose to obtain larger productivity of single furnace under lower investment, employ double row tubes, and for combined feature of both single and double row arrangements, employ staggered row tubes.
The radiant tubes employing single row arrangement in radiant section receive double-wall radiation; they have the most uniform heat receiving and best heat conducting effect. But the disadvantage is that in same area the number of tubes capable to be arranged is minimum, the productivity of specific area is low. Under this condition of single row arrangement, in order to meet the requirement of magnification of pyrolysis furnace, we have to extend the length of every radiant tubes and the length of radiant section, an in exorable result is that we have to greatly increase the height and length of radiant section and meet the more severe requirement for uniform heat supply by burners in radiant section At the same time, extremely long radiant tube makes the engineering problems complicated. Therefore, the use of single row arrangement structure significantly limits the productivity of pyrolysis furnace.
Although the use of double row of radiant tubes arrangement can increase the productivity by 70%, but the mutual overlap between tubes of double row seriously effects heat conductivity of heater wall from its radiation, this results in worse conductive effect from radiation. At the same time non-uniform heat receipt of radiant tubes brings about disadvantageous effect to cracking selectivity, run length and lifetime of radiant tubes.
Although the use of staggered row arrangement can partly increase productivity and uniformity of heat receipt, but in order to ensure uniformity of radiant heat conduction, the pitch between adjacent radiant tubes must be not lower than 1.8 times of outer diameter thereof, therefore, the space saved within furnace chamber is limited, Be walls, In order to avoid mutual cross-link of radiant tube bends in adjacent groups and manifolds in lower portion of furnace chamber, we have to locate the bends of adjacent groups and manifolds at different heights or different planes, this leads to two by-effects: at one hand, the radiant tubes, located at different heights, having different overall lengths in various groups, thus re retain time and severity of feedstock is different in various groups, this makes certain limitation to the optimization controlling; At the other hand, the bends and manifolds located at different planes give significant affectation to stress of all radiant tubes, which easy to cause deformation of radiant tubes. Moreover, this leads to a complicated design of radiant tube, bends and manifolds; an increased types of them, a bad interchangeability of them, high difficulty in mounting and increased investment.
FIG. 6 shows an arrangement of prior art within which two pass branched radiant tubes 7 with different diameters of type 2-1 are arranged in radiant section 3, wherein the first pass and second pass tubes are located in the same plane, that is a single row arrangement. It can be seen from the figure, although the tubes uniformly receive heat, but in total not so many tubes are arranged in radiant section, the space utilization ratio is not high, as well as the arrangement of tubes is not symmetrical and tube lengths are not the same, this leads to different working conditions of cracking process in various tubes, and thus the cracking effect is affected.
To sum up, although all the furnaces of prior art have their advantages, but also have many shortages and problems. Therefore, it is necessary to seek for a pyrolysis furnace of new type with excellent compositive properties, to overcome above described shortages.